display field value from child model in parent model tree view in odoo

Question

I want to display a value from the child model in the parent model tree view using one2many field in the parent model and computed field. my code

class PurchaseRequest(models.Model):
    _name = 'purchase.request'
    _description = 'Purchase Request'

    estimated_costs = fields.Monetary(
       string='Estimated Cost', compute='_compute_estimated_cost',
        help='Estimated cost of Purchase Request Line, not propagated to PO.', store=True, 
         readonly=True)
    line_ids = fields.One2many('purchase.request.line', 'request_id',
                               'Products to Purchase',
                               readonly=False,
                               copy=True,
                               track_visibility='onchange')

    @api.multi
    @api.depends('line_ids.estimated_cost')
    def _compute_estimated_cost(self):
        for move in self:
            estimated_costs = 0.0
            for line in move.line_ids:
                estimated_costs = line.estimated_cost

class PurchaseRequestLine(models.Model):
    _name = "purchase.request.line"
    _description = "Purchase Request Line"

    estimated_cost = fields.Monetary(
        string='Estimated Cost', currency_field='currency_id', default=0.0,
        help='Estimated cost of Purchase Request Line, not propagated to PO.')